Background: We have been interested in the molecular structural of the human immunoglobulin genes for many years. To understand these genes in fine structural detail we have examined both human heavy and light chain genes by molecular cloning, restriciton map analysis and nucleotide sequence determination. Those studies have allowed us to understand the molecular events involved in the formation of a functional immunoglobulin gene. During our analysis of the human lambda light chain gene system, we discovered a class of genes that on Southern genomic blot analysis showed weak homology to the lambda constant regions. Results: To determine the nature of these new genes, we isolated recombinant DNA clones that contained these regions of human lambda constant region homology. Fine structure analysis of these genes, including DNA sequence, revealed two genes which showed homology to lambda constant regions but differed by approximately 15% at the deduced amino acid sequence level. These two genes contained open reading frames that in size and structure could encode functional light chain proteins. Further analysis of these constant regions revealed a single J region element, including the heptamer and nonamer recombinational signal sequences, in front of each gene. Therefore, these genes could be functional immunoglobulin light chain genes, but to date have not been described lambda protein, these genes may encode a new class of human light chain proteins, or may represent an example of immunoglobulin genes that have been recruited for a new function. At present, we are continuing our studies of these genes by looking for their rearrangement and expression in human hematopoietic cells.